1. Field of the Invention
This invention relates to the field of power transmission systems for helicopters, and more particularly pertains to such systems supported on an airframe to permit pivotal rotation of the transmission, rotor and rotor controls about the roll and pitch axes, or any mutually perpendicular axes that intersect the yaw axes.
2. Description of the Prior Art
In conventional helicopters, as the forward speed of the aircraft increases, the disc plane is tilted so that the forward component of the rotor lift vector is directed in the direction of flight. Current rotor control systems include rotor blade cyclic pitch control, which cause the blades to flap cyclically as they rotate about the axes of the rotor. This cyclic flapping produces large dynamic loads, which adversely effect the service life of the mechanical components on which the loads are carried, and vibration, which adversely affects the comfort of the crew and passengers. In helicopters having tandem rotors, the control systems differentially tilt the rotors so that the thrust component of the lift vector has the magnitude and direction required to produce the hover yaw maneuver. The dynamic loads and vibrations produced during this maneuver are also large.
Conventional helicopter rotor systems have the rotor shaft fixed in position relative to the airframe. The controls and rotor change position relative to the airframe and rotor shaft when control system cyclic input is applied to change the direction of the thrust vector, the component of the rotor lift force that is directed parallel to the direction of forward flight.
Preferably rotor blade flapping, the related dynamic loads and vibration levels are reduced if the rotor shaft itself is permitted to pivot. If this were done, the size of the rotor hub and other dynamic components could be reduced and their service lives extended because lower rotor loads result.
In conventional rotor systems the inclination of the fixed rotor shaft is set at an angle that is satisfactory for hover and forward flight requirements, but the angle is not an optimum for both of these requirements. If the rotor shaft were free to pivot about mutually perpendicular axes, the fuselage of the aircraft would be disposed in relation to the direction of flight to lower the drag forces, which reduce forward speed and require greater power than is necessary if the fuselage were aligned with the direction of flight.